1. Field of the Invention
This invention relates to polyphase filters for wireless communication systems, and more particularly, to polyphase filters having adjustable phase for obtaining accurate quadrature in digital cellular telephone systems.
2. Related Art.
Modern wireless communication systems such as digital cellular telephone systems send and receive signals by encoding and decoding information on a radio frequency carrier with phase components that can be mapped on an I-Q plane. Such systems need high-accuracy phase splitters to produce accurate quadrature for modulation in radio transmitters. High accuracy phase splitters are also needed for demodulation in radio receivers. Such phase splitting is often accomplished through the use of polyphase filters.
Many polyphase filters only produce an accurate phase split over a narrow frequency range, and even polyphase filters with wide-band phase splitting characteristics are only as accurate as the matching of the electrical parameters of their passive components. Good matching requires devices that are large in size in the context of cellular telephones, for example, and suffer from increased parasitic losses that degrade noise performance and consume more power, shortening already limited battery life. Furthermore, other circuits in the radio receiver besides the phase splitter itself can alter the phase and introduce errors in quadrature. Such errors are typically small (on the order of 5 degrees), but industry requirements are on the order of 2 to 3 degrees or less, so even small errors can present a problem.